1. Field of the Invention
This invention relates generally to loudspeaker ports. More particularly, the invention relates to providing a method for predicting the performance of a loudspeaker port based on the modeling and analysis of bi-directional fluid flow through the loudspeaker port.
2. Related Art
Bass reflex ports are used in loudspeakers to enhance low frequency performance. Over the last few years, there has been increased interest in bass reflex ports driven by the need for better performance from smaller loudspeaker enclosures, i.e., higher maximum sound pressure level and wider bandwidth. Although there has been significant work done to reduce these negative effects, no optimal solution has been found.
At low sound levels the port extends the low frequency response by supplying one of the components of a Helmholtz resonator. However, at higher sound levels, the turbulent intensity in the port increases, which disrupts the Helmoholtz resonance and causes distortion, noise and compression. To eliminate the distortion, noise and compression occurring at higher sound levels, many studies have been conducted in an attempt to understand what causes the instability and optimize port design.
For example, in one study, it was suggested that a symmetrical port with a flange and a blend radius at each end was optimum. Another study concluded that a gently flared port with small radius is the optimum configuration to avoid unwanted effects. A more recent study concluded that ports with generous flares performed best at low sound levels, and straight ports performed best at very high levels. Based upon this conclusion, the study suggested that ports with moderate flares represented the best compromise and performed well over a wide range of sound pressure levels. This study also noted that port flow is bi-directional. Accordingly, both entrance and exit loses should be considered since both effect the performance of the port. Because this study only utilized data based upon unidirectional flow principles but recognized the port flow at bi-directional, the study found port symmetry to be an important design consideration.
Although with these studies, several conclusions were also made about what causes port instabilities, as well as, where and when instabilities incur. One such study established that an over-driven port is more erratic in nature than a port driven in the linear region. This study proved that instabilities start to occur when port velocities approach the 5 to 10 m/s range and that ports display hysteresis effects. Thus, once instability occurs in a port the drive level must be lowered significantly to restore stability.
Another study concluded that undesirable blowing noises are caused by boundary layer turbulence and unsteady separation of the acoustic flow at the port termination. From a numerical simulation of the flow through a port, the study further illustrates that vortices occur at the port termination for straight ports and closer to the center with more generously flared ports.
In reaching conclusions regarding port performance and optimal port designs, researcher based their conclusion upon port performance measured using either theory or experimentation based upon the characteristic of air or fluid through the port. It is well established that fluid flow characteristic in and around a port can be used to analyze and model port performance.
The flow in and around a loudspeaker port can be described as a complex high-speed oscillating acoustic flow of varying magnitude and frequency due to the forces created by the loudspeaker's transducer. When there is a combination of low frequency and high magnitude, flow conditions become such that areoacoustically generated noise can become a problem. If the flow within the port is thought of as a simple pipe flow problem, it is important to recognize that as the flow changes direction the inlet and outlet reverse their roles. Thus, the flow direction within a port can be characterized as either bi-directional or oscillating.
Although port flow is oscillating, unidirectional fluid flow principles are generally used to analyze and model port performance. While entrance and exit loses for unidirectional flows within a pipe are well documented, oscillating flows are not. Unidirectional flow principles demonstrate that if a pipe's inlet has a sharp edge, flow separation can be expected in a phenomenon known as “vena contracta.” This flow separation can, however, be avoided by rounding the edge. Generally, the greater the radius, the less the loss. Further, a small amount of rounding can make a significant difference. Although entrance losses are heavily associated with geometry exit losses are not. Since flow separation is inevitable at the exit, it would be optimum for the flow to separate at the port termination. Using a straight pipe with a sharp edge provides the best opportunity for this to occur. As a result the optimum geometries for entrance and exit are significantly different based upon the principle of unidirectional flow.
Since port flow is bi-directional, entrance and exit geometry must be considered together to find an optimal profile. Oscillating or bi-direction flow does not follow many of the standard rules that govern unidirectional flows. While it is important to examine and understand the optimal inlet and outlet for unidirectional flows, to accurately model port flow to analyze and optimize port designs, bi-direction flow principles should also be utilized.
Accordingly, a need exists for a method for predicting port performance and optimizing port design using bi-directional fluid flow principles.